In radio frequency ("RF") transmission systems that utilize amplitude modulation ("AM") techniques to transmit information, an audio signal containing the information is used to modulate the amplitude of an RF carrier signal. Currently, amplitude modulation is performed by translating the frequency components of the audio signal to occupy a different position in the frequency spectrum. This is accomplished by multiplying a time-variant function that describes the carrier, typically an RF sine wave, with a modulation signal, typically containing some useful data or information and having no frequency components greater than one-half the carrier frequency. Multiplying one sinusoidal waveform (A sin w.sub.A t) by another (B sin w.sub.B t) results in an output frequency composition that may be represented by the following equation: EQU A sin w.sub.A t*B sin w.sub.B t=(AB/2)[cos(w.sub.A -w.sub.B)t-cos(w.sub.A +w.sub.B)t] (1)
Half of the output power ("Power.sub.out ") is at a frequency equal to the sum of the carrier and the modulation signal. The other half is at a frequency equal to the difference in the frequencies. Therefore, the output power may be represented by the following equation: EQU Power.sub.out =1/2g(f+c)+1/2g(f-c) (2)
where g(f) is the modulation signal and c is the carrier signal. This result is represented graphically in FIG. 1A.
In practice, the process of amplitude modulation is enhanced by adding a DC component to the modulation signal, which results in the addition of the carrier, at a constant level A.sub.0, to the output spectrum. This spectrum may be represented by the following equation: EQU A.sub.0 +[(A sin w.sub.A)*(B sin w.sub.C)]=A.sub.0 cos w.sub.C +AB/2[cos(w.sub.A -w.sub.C)-cos(w.sub.A +w.sub.C ] (3)
Again, the sum and difference signals are equal, but now there is also power allocated to the carrier. Therefore, the output power may be represented by the following equation: EQU Power.sub.out =A.sub.0 c+1/2g(f+c)+1/2g(f-c) (4)
where g(f) is the modulation signal and A.sub.0 c is the carrier signal. This result is illustrated graphically in FIG. 1B.
All of this occurs because the modulation process is continuous; that is, at all angles of the carrier's cycle, its amplitude is being modulated. This is what creates the side-bands g(f-c) and g(f+c).
The amplitude modulation process described above suffers from several deficiencies. In particular, the bandwidth needed is typically twice the bandwidth of the modulation signal and, at best, equal to the bandwidth of the modulation signal. Moreover, a portion of the energy of the output signal contains no modulation. Of all the energy employed, only a limited amount actually carries information and the remainder is unused.
Therefore, what is needed is a technique for amplitude modulating that remedies the deficiencies of the prior art as described above.